Combustion of fuel sources such as coal produces a waste gas, referred to as “flue gas” that is to be emitted into an environment, such as the atmosphere. The fuel sources typically contain sulfur and sulfur compounds which are converted in the combustion process to gaseous species, including sulfur oxides, which then exist as such in the resulting flue gas. The fuel sources typically also contain elemental mercury or mercury compounds which are converted in the combustion process to, and exist in the flue gas as, gaseous elemental mercury or gaseous ionic mercury species.
As such, flue gas contains particles, noxious substances and other impurities considered to be environmental contaminants. Prior to emission into the atmosphere via a smoke stack, hereinafter a “stack”, the flue gas undergoes a cleansing or purification process. In coal combustion, one aspect of this purification process is normally a desulfurization system, such as a wet scrubbing operation commonly known as a wet flue gas desulfurization (WFGD) system.
Sulfur oxides are removed from flue gas using a WFGD system by introducing an aqueous alkaline slurry to a scrubber tower of the WFGD system. The aqueous alkaline slurry typically includes a basic material that will interact with contaminants to remove them from the flue gas. Examples of basic materials that are useful in the aqueous alkaline slurry include lime, limestone, magnesium, combinations thereof and the like.
Recently, there has been an increased focus on the removal of mercury. Presently, there are various methods for removing mercury from flue gas. Those methods include addition of oxidizing agents in a boiler upstream of the flue gas emission control system and then removing the same with scrubbers, addition of reactants to bind mercury and removing the same from the flue gas, and utilization of particular coal or fuel to minimize the amount of mercury released when the coal or fuel is burned.
A number of generally known methods of mercury removal are effective to produce mercury salts, which can be dissolved and removed by the aqueous alkaline slurry used in the wet scrubbing operation. Some of these methods include the addition of halogen or halogen compounds, such as bromine, to the coal or to the flue gas upstream of the wet scrubbing operation to provide oxidation of elemental mercury to ionic mercury and formation of mercury salts, which are then dissolved in the aqueous alkaline slurry incident to the sulfur oxide removal processes. However, the removal of mercury in the aqueous alkaline slurry of a wet scrubber has proven to be difficult to control and it is not easily predicted when designing a flue gas cleaning system with respect to mercury removal. The desired emission guarantee levels are often as low as 0.3 μg/Nm3 of mercury, which corresponds to a very high mercury removal efficiency in the wet scrubber.